Cross-linking polyethylene compositions with silicone additive

ABSTRACT

A CURABLE COMPOSITION, ESPECIALLY ADAPTABLE FOR USE AS INSULATION FOR WIRE AND CABLE, COMPRISING AN ETHYLENECONTAINING POLYMERIC MEMBER, A CURING AGENT, AND A SILICONE FLUID HAVING A VISCOSITY NOT GREATER THAN ABOUT 100 CENTISTOKES AT 25*C. A FILLER SUCH AS CARB~N BLACK OF A MINERAL FILLER MAY BE INCORPORATED INTO THE SYSTEM.

United States Patent ()1 hce 3,795,646 Patented Mar. 5, 1974 No. 794, Jan. 5, 1970. This application Apr. 21, 1972, Ser. No. 246,153

Int. Cl. C08g 51/04, 51/22 US. Cl. 260-291 SB 20 Claims ABSTRACT OF THE DISCLOSURE A curable composition, especially adaptable for use as insulation for wire and cable, comprising an ethylenecontaining polymeric member, a curing agent, and a silicone fluid having a viscosity not greater than about 100 centistokes at 25 C. A filler such as carbon black or a mineral filler may be incorporated into the system.

This application is a continuation of our then co-pending, and now abandoned, application Ser. No. 794, filed Jan. 5, 1970, entitled Curable Eethylene Compositions Containing an Organic Silicone Additive.

This invention relates to an improved insulating composition. In its more specific aspect, this invention relates to an ethylene polymeric composition useful as insulation characterized by improved ionization resistance, and to electric wire and cable insulated with such a composition.

Thermosetting or cross-linked polyolefin compositions, such as polyethylene, are well known and have been used extensively, especially for insulation materials for wire and cable. In the conventional manufacture of wire and cable employing such insulation materials, the curing agent, filler and/or other additives are incorporated into the polymer. The compounded admixture is subsequently fabricated over a metallic conductor as an insulation coating and then cured at an elevated temperature to form a thermosetting or cross-linked coating. Where desired, the curing agent may be omitted, and the fabricated coating can be cross-linked by irradiation. Cross-linked polyolefins are gaining wide use elsewhere such as for expandable tube for splicing, for molded parts and for laminates.

Cross-linked ethylene-containing polymeric compositions as a general rule exhibit excellent physical properties and electrical properties such 'as a high hot tensile strength and a low dielectric losses, but this might be varied depending upon other ingredients. The growth of this material as cable insulation has been rapid, but more recent demands for higher voltage cable require even superior properties because of the high voltage stress placed upon the cable. In power cable applications for transmitting relatively high voltage loads such as kv. and above, ionization resistance, sometimes referred to as corona resistance, may become a problem because it may lead to the premature breakdown of the cable insulation. However, known cross-linked polyethylene compositions, for example, have an ionization resistance level which is observed under a high voltage stress. Actually, the ionization resistance values for these compositions are extremely erratic in that the same sample can exhibit widely divergent values. Consequently, an average value is usually cited for the particular composition. Various additives to the polymeric systems for the purpose of improving ionization resistance have been investigated, but have met with little or no success. Most notably, these additives have included silicone gumsor rubbers which have a relatively high viscosity, i.e., 1,000,000 centistokes at 25 C. In the polymeric systems, the silicone gums are curable and have substantially no effect on ionization resistance unless used in relatively large quantities, e.g., 15 parts per parts of polymer, and have an adverse eifect on the mixing characteristics.

This invention has therefore as its object to provide ethylene-containing polymeric compositions which exhibit improved ionization resistance under high voltage stress while not adversely effecting or altering the other properties of the composition. Other objects and advantages of the invention will become evident from the following description.

In accordance with the broad aspect of this invention, there is provided a curable composition, especially useful as insulating material for wire and cable, comprising an ethylene-containing polymer, a curing agent for eifecting cross-linking of the polymer, and an organic silicone fluid having a viscosity not greater than 100 centistokes at 25 C. Where desired, a filler such as carbon black or a mineral filler may be incorporated into the polymeric system. In preparing the composition, the polymer, silicone fluid, and filler and other additives, where desired, are intimately admixed as in a Banbury at an elevated temperature sutficient to flux or plasticize the polymer. A suitable curing agent, desirable a tertiary peroxide, is then incorporated into the admixture, and blending is continued below the incipient cure temperature of the curing agent. The composition is subsequently fabricated as by molding or as by extrusion over a conductor to provide an insulation layer, and then cured to form a thermosetting or cross-linked product. In the manufacture of cable, it is conventional to pass the extruded product through a curing oven operated under steam pressure of about 225 to 250 p.s.i.g. Where cross-linking of the polymer takes place. The cured product exhibits substantially improved ionization resistance under a high voltage stress without any adverse effect or loss in the other properties, which is discussed herein below in greater detail.

The terms wire and cable are used herein and in the appended claims as synonymous terms to refer to insulated conductors.

It was unexpected to find that the incorporation of a small amount of'the silicone fluid into the polymeric 'systemenhanced considerably the ionization resistance of the cured product. Moreover, the polymeric system made in accordance with the present invention yields relatively consistent ionization resistant values as compared to erratic and extremely divergent measurements obtained on substantially the same polymeric system but containing no such silicone fluid. Consequently, the measurements on compositions of our invention are more meaningful. The silicone fluid should have a viscosity not greater than about 100 centistokes at 25 C., and preferably not greater than about 50 centistokes, because there is a rather sharp decline in the improvement obtained with higher viscosity silicone fluids, and soon reaches a point where there iS little or no improvement. Silicone fluids having a viscosity from about 5 to 100 centistokes are especially useful, and cross-linked polyethylene, for example, containing this silicone fluid have shown an increase in ionization resistance by a factor greater than 10 and sometimes 20. The amount of silicone fluid incorporated into the polymeric system should be sufficient to result in a marked improvement in ionization resistance, but this may vary somewhat depending upon the particular polymer and filler. In general, the amountof silicone fluid should be at least 2 parts by weight per 100 parts polymer, although as little as 1 part by weight has shown substantial beneficial results. Ethylene-containing polymeric systems containing from about 2 parts to 5 parts by weight silicone fluid per lOO parts of polymer, and preferably about 2 to 3 parts, are especially suitable, and there. appears to be no beneficial effect in using increased amounts, although 3 this may depend somewhat upon the particular polymer and filler.

The silicone fluids used in this invention are well known and are frequently referred to as organopolysiloxanes having a general structural formula where R is an organic radical such as a lower alkyl, carboxyalkyl, hydroxyalkyl, alkenyl, aryl, and aralkyl radicals, and n is the number of units in the chains. In the preferred embodiment, R is a methyl and/or phenyl radical and the silicone fluid has from about 1.9 to 2.1 silicombonded organic groups per siliconatom. Suitable silicone fluids include, for example, dimethylpolysiloxane, methylphenylpolysiloxane, and methylvinylpolysiloxane.

The curable polymer included within the scope of the present invention comprises an ethylene-containing polymeric member selected from the group consisting of polyethylene, blends of polyethylene and other polymers, and copolymers of polyethylene and other polymerizable materials. Suitable copolymers include, for example, ethylenevinyl acetate, ethylene-propylene copolymer and ethylenepropylene ter-polymer. The polyethylene may be blended, for example, with vinyl acetate, ethylacrylate, propylene, butene-l as well as with other copolymers such as ethylene-propylene or ethylene-vinyl acetate. The blend or copolymer should comprise not less than about 50% by weight ethylene, and more generally about 70 to 90% by weight ethylene, the balance being the other polymeric material, but this will depend laregly upon the properties desired for the end product. Although the invention is described hereafter with specific reference to polyethylene, it should be understood that blends or copolymers of ethylene are also useful.

Although the invention has been described with particular emphasis on filled polymeric systems, it should be understood that the invention is also intended to include unfilled systems as well. Thus, both filled and unfilled cross-linked polyethylene systems have been employed as insulation for high voltage cable, and it has been found that the incorporation of a small amount of silicone fluid in accordance with the present invention substantially improves the corona resistance of the cured product. Where desired, a suitable filler is compounded with the polymer and other additives. The fillers utilized in the composition may be any of those commonly employed in polymeric compositions and include the various carbon blacks and mineral fillers. The mineral fillers useful in the invention include, for example, aluminum silicate, aluminum oxide, calcium silicate, silica, magnesium silicate, titanium dioxide and mixtures thereof. These filler materials are well known and readily available on the market, and the type of filler used will depend largely on the desired properties for the product and may be determined by one skilled in the art. The function of fillers in these polymeric systems is well known, and the amount of filler incorporated into the composition may be varied depending upon the properties desired in the cured product. The filler content may range from about 20 to 50% by Weight of the composition, and more preferably about 25 to 40% by weight.

The compounding operation is conducted within a temperature range high enough to render the admixture sufficiently plastic to work, but below the reacting temperature or decomposition temperature of the curing agent so that the curing agent will not decompose thereby causing at least partial or incipient curing of the polyethylene stock during the normal mixing cycle. Desirably, the curing agent employed in the operation is an organic peroxide,

such as a tertiary peroxide, and characterized by at least one unit of the structure which decomposes at a temperature in excess of about 295 F. The use of these peroxide curing agents in effecting cross-linking of polymers such as polyethylene compounds is adequately described in US. Pats. 3,079,370, 2,888,424, 3,086,966 and 3,214,422, which patents are incorporated in this specification by reference. The most commonly used peroxide curing agent, and the agent preferred, is di-a-cumyl peroxide. Other useful curing agents include the tertiary diperoxides such as 2,5-dimethyl-2,5- di(t-butyl peroxy) hexane and 2,5-dimethyl-2,5-di(t-butyl peroxy) hexyne-3, and the like diperoxy and polyperoxide compounds.

The proportion of peroxide curing agent used depends largely on the mechanical properties sought in the cured product, for example, hot tensile strength. A range of from about 0.5 to 10 parts by weight of peroxide per hundred parts of polymer satisfy most requirements, and the usual proportion is of the order of two to four parts peroxide. In a typical production operation employing a tertiary peroxide as a curing agent, compounding is conducted at a temperature of from about 200 to 275 F. If compounding is conducted at a temperature much higher than the stated maximum, the peroxide will decompose thereby causing premature curing of at least a portion of the polymer. As a consequence, the compound will be difficult to fabricate and the final product will exhibit an irregular or roughened surface.

Also, certain additives whose function are well known are usually compounded with the polymeric blend. These compounding additives include, for example, an antioxidant such as polymerized trimethyldihydroquinoline, a lubricant such as calcium stearate to prevent the composition from sticking during fabrication, a non-combustible additive such as antimony oxide to provide flame retardance, a coagent such as polybutadiene to facilitate cross-linking, and a small amount of pigment or coloring agent. The compounding additives required may vary considerably, and there may be others than those mentioned depending upon the properties sought for the end product. These additives are normally admixed with the polymer and filler as in a Banbury to form a substantially homogeneous admixture.

In the following examples which further illustrate the invention, the samples were prepared, including the controls containing no silicone fluid, on a two-roll rubber mill according to the following procedure: the polymer, filler, silicone fluid and other additives were first admixed on the rubber mill at a temperature of about 225 F. for about 4-5 minutes. The di-u-cumyl-peroxide curing agent was added and mixing continued for another 2 minutes. The compounded composition was then stripped from the mill and cooled to room temperature. For each compound, panels measuring 6 in. x 6 in. x 10 mils were molded between parallel platens for twenty minutes at 350 F. under a pressure of tons thereby cross-linking the panels. The press was then cooled to room temperature, and the molded panel removed. A sample two inches in diameter was cut from each molded panel. The recipes for the samples are shown in Table I below for Examples 1-9.

Each sample was tested for corona resistance according to the procedure set forth in A.I.E.E. Transactions, Progressive Stress-A New Accelerated Approach to Voltage Endurance, W. T. Starr and H. S. Endicott, -vol. 80, 1961, part III, Power Apparatus and Systems. The test apparatus was modified to provide a fuse wire extending from the bottom electrode to ground so that when the 5 sample failed, the fuse wire broke. The ionization resistance is shown in Table I below for Examples 1-9.

ple prepared in accordance with the invention shows an increase in ionization resistance for the latter.

TABLE I.-SILICONE FLUIDS F VARYING VISCOSITIES IN POLYETHYLENE SYSTEMS (centistokes at 25 C.)

Example number Polyethylene Aluminum silicate Polymerlzed tri-methyldihydroquinoline (antioxidant) Di-a-cumylperoxide 90% active (curing agent) Dim ethylpolyslloxane, centistokes:

- non Control (no silicone fluid) Ionization resistance (500 volts per mil average stress) From the table it will be observed that silicone fluids having viscosities no greater than 10 centistokes substantially improve the ionization resistance of the cured product. As shown, a cured sample containing no silicone fluid had an average ionization life of 369 minutes whereas the addition of three parts of silicone fluid having a viscosity of S0 centistokes showed an ionization life greater than 5800 minutes.

Table II, below, shows the results obtained with varying concentrations of dimethylpolysiloxane; and Table II, below, shows the results obtained with an unfilled polymeric system and polymeric systems with different fillers.

TABLE II.CONCENTRATION EFFECT What we claim as new and desire Patent of the United States is:

1. A curable composition having improved resistance to corona consisting of:

(a) a polymeric member containing not less than abou by weight of ethylene selected from the group consisting of polyethylene, blends of polyethylene and other polymers, and copolymers of ethylene and other polymerizable materials selected from the group consisting of propylene and vinyl acetate; (b) a curing agent for eflecting cross-linking of said polymeric member; and (c) a silicone fluid having a viscosity not greater than about 1-00 centistokes at 25 C. present in the range of from about 2 to 5 parts by weight per100 parts by weight of polymeric member. 2. The cured product of claim 1. 3. The composition according to claim 1 wherein said polymeric member is polyethylene.

4. The composition according to claim 1 wherein said polymeric member is ethylene-propylene.

to secure by Letters TABLE III.FILLED .AND UNFILLED CROSS-LINKED POLYETHYLENE SYSTEMS Example number 14 14A 15 15A 16 16A 17 17A 18 18A.

Polyethylene 100 100 100 100 100 100 100 100 100 100 Carbon black 40 40 Magnesium silicone 30 30 Hydrated alumina Titanium dinxirle Polymerized trl-methyldihydroquinoline (antioxidant) 1 1 .5 1.5 .5 .5 .5 .5 .5 .5 Di-a-cumylperoxide 90% active (curing agent) 2 2 3.11 3.11 3.5 3.5 3.5 3.5 3.5 3.5 Dimethylpolysiloxane:

(20 est. 25 C. 3

In Table III, for each example, the number without the letter is the control and the number followed by the letter A included the silicone fluid. Thus, the addition of the silicone fluid substantially increased the ionization resistance of the cured polymer.

Table IV, below, illustrates the beneficial results obtained when using copolymers.

Here, again, a comparison of the controls with the sam- 5. The composition according to claim 1 wherein said polymeric member is ethylene-vinyl acetate.

6. The composition according to claim 1 wherein said silicon fluid has a viscosity of about 50 centistokes at 25 C.

7. The composition according to claim 1 wherein said curing agent is an organic peroxide.

8. A curable composition having improved resistance to corona consisting of polyethylene, an organic peroxide curing agent, and a silicon fluid having a viscosity not greater than centistokes at 25 C. and presentin the range of about 2 to 5 parts by weight per 100 parts by weight of polyethylene.

9. The cured product of claim 8.

10. The curable composition according to claim 1, wherein the silicon fluid is dimethylpolysiloxane.

11. A method of improving corona resistance in cured, ethylene containing polymeric compositions for electrical insualtion, comprising admixing a silicone fluid having a viscosity of not greaterthaii about 100 centistokes at 25 C. with a curable polymeric composition containing not less than about 5 by weight polymeric ethylene selected from the group consisting of polyethylene, blends of polyethylene and other polymers, and copolymers of ethylene and other polymeriza'ble materials selected from the group consisting of propylene and vinyl acetate, and a curing agent for eflectin-g a cross-linking cure of the ethylene containing polymeric composition, said silicone fluid be ing admixed with the curable polymeric composition in amount of from about 2 to 5 parts by weght per 100 parts of polymeric composition; and curing the ethylene containing polymeric composition.

12. The method of claim 11, wherein the silicone fluid is selected from the group consisting of'dimethylpolysiloxane, methylphenylpolysiloxane and methylvinylpolysiloxane.

13. The method of claim 11, wherein the silicone fluid is admixed with the curable polymeric composition containing filler.

14. The method of claim 13, wherein the filler content is about 20 to 50% by weight of the composition.

15. Th e method of claim 14, wherein the curable polymeric composition is selected from the group consisting of polyethylene, ethylene-vinyl acetate, ethylenepr'opylen'e copolymer, and ethylene-propylene terpolymer.

16. Thqmethod of claim 15, wherein the silicone fluid has a viscosity of not greater than about 50 centistokes I at25 ,C.

ethylene-propylene copolymer, and ethylene-propylene terpolymer, and a tertiary peroxide curing agent 'fo'retfecting a cross-linking cure of ethylene containing polymeric composition, said silicone fluid being admixed with the curable polymeric composition in amount of from about 2 to 5 parts by weight per parts of polymeric composition; and curing the ethylene containing polymeric composition. I

19. The method of claim 18, wherein the silicone-fluid is admixed with the curable polymeric composition containing at least one filler selected from the group consisting of aluminum .oxide, calcium silicate,'silic'a,- magnesium silicate, and titanium dioxide.

20. A method of improving corona resistance in curable ethylene containing polymericcompositions, comprising admixing a silicone fluid having a viscosity of not greater than about 100 centistokes at 25 C. with a curable polymeric composition comprising at least one polymeric material containing not less than about.5 0%iby weight of polymeric ethylene selected from the groupzconsisting of polyethylene, ethylene vin'yl acetate, ethylene-propylene copolymer, and ethylene-propylene terpolymer, and a curing agent for eflecting'a cross-linking cure of'the ethylene containing polymeric composition, said silicone fluid being admixed with the curable polymeric composition in amount of from about 2 to 5' parts by'weight per 100 parts of polymeric composition.

References Cited UNITED STATES PATENTS;

; 3,148,169 9/1964 Martens et al. 2605-4111 2,888,419 5/1959 Safford -11-- -260--29:, 1SB 2,930,083 3/1960 Vostovich et al.- 2647174 VUX 3,171,866 3/1965 Meyer et al. 117 -232 X 3,425,983 2/1969 Wolfe 260-415 R LEWIS T. JACOBS, Primary Examiner I 

